ISO 18191:2015

Water quality — Determination of pHt in sea water — Method using the indicator dye m-cresol purple

ISO 18191:2015 specifies a spectrophotometric determination of the pHt of sea water on the total hydrogen ion concentration pH scale using the indicator dye m-cresol purple. The total hydrogen ion concentration, [H+]t, is expressed as moles per kilogram of sea water. The method is suitable for assaying oceanic levels of pHt 7,4 to 8,2 for normal sea water of practical salinity ranging from 20 to 40.

Qualité de l'eau — Détermination du pHt dans l'eau de mer — Méthode utilisant l'indicateur coloré au pourpre de m-crésol

General Information

• Status: Published
• Publication Date: 31-Aug-2015
• ICS: 13.060.50 - Examination of water for chemical substances
• Technical Committee: ISO/TC 147/SC 2 - Physical, chemical and biochemical methods
• Drafting Committee: ISO/TC 147/SC 2 - Physical, chemical and biochemical methods

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 18191
First edition
2015-09-01
Water quality — Determination of
pH in sea water — Method using the
t
indicator dye m-cresol purple
Qualité de l’eau — Détermination du pH dans l’eau de mer —
t
Méthode utilisant l’indicateur coloré au pourpre de
m-crésol
Reference number
©
ISO 2015
© ISO 2015, Published in Switzerland
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ii © ISO 2015 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Terms and definitions . 1
3 Principle . 2
4 Reagents . 2
5 Apparatus . 3
6 Sampling . 4
7 Procedure. 4
8 Calculation and expression of results . 4
8.1 Correction of measured absorbances . 4
8.2 Calculation of the pH of the sea water and indicator . 5
t
8.3 Correction for pH change resulting from addition of the indicator . 6
t
Annex A (informative) Performance data . 7
Annex B (informative) Storage stability . 8
Bibliography .10
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
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electrotechnical standardization.
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described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
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Barriers to Trade (TBT) see the following URL: Foreword - Supplementary information
The committee responsible for this document is ISO/TC 147, Water quality, Subcommittee SC 2, Physical,
chemical and biochemical methods.
iv © ISO 2015 – All rights reserved

Introduction
The greenhouse effect induced by anthropogenic carbon dioxide, CO , in the atmosphere is one of the
serious global environmental issues. A key factor controlling the atmospheric CO is its absorption into
the ocean. As a result of the absorption, the pH in the upper layer of the ocean is observed to have fallen
gradually, and its influence on the living organisms is a matter of concern all over the world.
On the other hand, carbon capture and storage (CCS) technology is considered as a useful means of
reducing the CO emissions from fossil fuel. When ocean environment such as sub-seabed aquifer
is selected as a storage site, the monitoring of carbonate system including pH in sea water becomes
very important. The analytical method for pH in sea water (the total hydrogen ion concentration
t
pH scale) samples requires specific conditions and techniques essential to the precise and accurate
determination. This International Standard describes a method for the determination of pH in sea
t
water with the repeatability less than 0,003.
This method will provide international communities accurate data sets on pH in sea water being
t
compatible with each other. This is the base of national and international operational observation or
monitoring programs of the oceanic carbonate system as well as individual research works.
INTERNATIONAL STANDARD ISO 18191:2015(E)
Water quality — Determination of pH in sea water —
t
Method using the indicator dye m-cresol purple
WARNING — Persons using this International Standard should be familiar with normal laboratory
practice. This International Standard does not purport to address all of the safety problems, if
any, associated with its use. It is the responsibility of the user to establish appropriate safety and
health practices and to ensure compliance with any national regulatory conditions.
IMPORTANT — It is absolutely essential that tests conducted in accordance with this
International Standard be carried out by suitably qualified staff.
1 Scope
This International Standard specifies a spectrophotometric determination of the pH of sea water on the
t
total hydrogen ion concentration pH scale using the indicator dye m-cresol purple. The total hydrogen
+
ion concentration, [H ] , is expressed as moles per kilogram of sea water. The method is suitable for
t
assaying oceanic levels of pH 7,4 to 8,2 for normal sea water of practical salinity ranging from 20 to 40.
t
2 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
2.1
+
total hydrogen ion concentration [H ]
t
hydrogen ion concentration including the contribution of the hydrogen sulfate ions in the sea water
Note 1 to entry: Total hydrogen ion concentration is defined as:
++ +−
[]HH=+[] 1 SH/[K ≈+][HSO ]
()
tF TS F 4
where
+
[H ] is the free concentration of hydrogen ion in sea water;
F
S
T
−−2
   
is the total sulfate concentration HSOS+ O ;
( 44 )
   
K
−
S
is the acid dissociation constant for HSO .
The pH is then defined as the negative of the base 10 logarithm of the hydrogen ion concentration as:
t
+
 
[]H
t
pH =−log  
t 10
 
mol/kg
 
2.2
practical salinity
S
ratio K of the electrical conductivity of the sea water sample at the temperature of 15 °C on IPTS-68
and the pressure of one standard atmosphere, to that of a potassium chloride (KCl) solution, in which
-3
the mass fraction of KCl is 32,435 6 × 10 , at the same temperature and pressure
3 Principle
The values of pH are determined by adding an indicator to sea water. For the sulfonephthalein
t
indicators such as m-cresol purple, the reaction of interest at sea water pH is the second dissociation as
t
given in Formula (1):
−+ 2−
HI aq =HaqI+ aq (1)
() () ()
where I represents the indicator, which is present at a low level in a sea water sample. The total hydrogen
ion concentration of the sample can then be determined as given in Formula (2):
2−
 
I
−  
pH =pHK I +log (2)
t () 10
−
 
HI
 
The principle of this approach uses the fact that the different forms of the indicator have substantially
different absorption spectra. Thus the information contained in the composite spectrum can be used to
2- -
estimate [I ]/[HI ].
At an individual wavelength, λ, the measured absorbance, A , in a cell with a pathlength L is given by
λ
the Beer-Lambert law as:
A
λ −− 22−−
   
=εεHI HI + II ++B e (3)
() ()
λλ λ
   
L
where B corresponds to the background absorbance of the sample and e is an error term due to
λ
- 2-
instrumental noise. Provided that the values of the extinction coefficients: ε (HI ) and ε (I ) have been
λ λ
measured as a function of wavelength, absorbance measurements made at two or more wavelengths
2- -
can be used to estimate the ratio [I ]/[HI ].
In the case that only two wavelengths are used, and provided that the background can be eliminated
effectively by a subtractive procedure, Formula (3) can be rearranged to give (assuming no
instrumental error):
2− −−
 
I AA//−εεHI HI
() ()
12 12
 
= (4)
− 2−− 2−−
 
HI εεIH//I − AA ε IH/ε I
() () () ()
1 21 2 22 2
 
where the numbers 1 and 2 refer to the wavelengths chosen. For the best sensitivity, the wavelengths
2- -
corresponding to the absorbance maxima of the base (I ) and acid (HI ) forms, respectively, are used.
The various terms ε are the extinction coefficients of the specified species at wavelengths 1 and 2,
respectively.
4 Reagents
Use only reagents of recognized analytical grade.
4.1 m-cresol purple, containing no spectrophotometrical impurities.
NOTE 1 Reference [14] showed that the indicator can be characterized and purified using the HPLC system.
- 2- - 2-
The wavelength of isosbestic point for HI /I of the pure m-cresol purple, λ (HI /I ) depends on the following
isos
- 2- -
formula: λ (HI /I ) = 490,6 – 0,10 t, where t is the temperature in degrees Celsius. That for H I/HI , it is also
isos 2
-
λ (H I/HI ) = 482,6 – 0,10 t.
isos 2
NOTE 2 References [14] and [17] describe the purification method of m-cresol purple.
2 © ISO 2015 – All rights reserved

4.2 Solution of pure m-cresol purple.
A concentrated (at least 2 mmol/l) pure indicator solution of known pH adjusted to be in the range
7,9 ± 0,1 pH units, chosen to match pH measurements from an oceanic profile, is required; this implies
t
that for m-cresol purple A /A approximately 1,6.
1 2
NOTE The absorbance ratio of a concentrated indicator solution can be measured using a cell with a short
pathlength (0,5 mm).
4.3 Deionized ultrapure water, of resistivity about 18 MΩ cm.
5 Apparatus
Usual laboratory equipment and, in particular, the following:
5.1 Flexible drawing tube
Approximately 40 cm long, sized to fit snugly over cell port. Silicone rubber is suitable. The drawing
tube can be pre-treated by soaking in clean sea water for at least one day. This minimizes the amount of
bubble formation in the tube when drawing a sample.
5.2 Spectrophotometric cell
These should be made of optical glass with a 10 cm pathlength, two ports and polytetrafluoroethylene
stoppers. A sufficient number of cells are needed to collect all the samples that will be analysed from a
particular cast.
NOTE A flow through cuvette with a 10 cm pathlength is also available. Sample bottles of at least 200 ml
with air tight caps are needed to use the cuvette.
5.3 Micropipette
A micropipette is used to add the indicator to the cell. It should be of ~0,1 cm capacity with a narrow
polytetrafluoroethylene (PTFE) tube attached to act as a nozzle.
5.4 High-quality spectrophotometer
For work of the highest sensitivity and precision, a double-beam spectrophotometer is desirable.
However, good results can be obtained with a high-quality single-beam instrument.
5.5 Temperature-control system for spectrophotometer cell
Commercially manufactured, thermostated spectrophotometer compartments that can accommodate
10 cm cells are rarely available and one will probably have to be custom-made. The temper
...